Fluoropolymers have generally excellent suitability for electrical applications since they present a low dielectric constant and a low dissipation factor, but they are quite expensive. Hence it is desirable to minimize the amount of insulated material in providing a foaming fluoropolymer composition.
In addition, even at high temperatures, fluoropolymers are stable and do not burn, and they can therefore in principle be used not only for electrical purposes but in all fields of engineering.
However no chemical foaming agent providing a decomposition temperature superior to the melting point of fluoropolymer exists on the market.
Document U.S. Pat. No. 6,064,008 proposes a flame retardant communication cable comprising elongated electrical conductor surrounded by a layer of insulating material, said insulating material comprising a chemically blown fluorinated polymer having a melting point of greater than about 248° C.
The insulated material is foamed using a chemical foaming agent, which decomposes at a temperature above the temperature needed to melt the fluorinated polymer and the gas evolved from the chemical foaming agent.
A particularly suitable chemical foaming agent is the barium salt of 5-phenyltetrazole.
However such chemical foaming agent presents a decomposition temperature below the fluorinated polymer having a melting point greater than about 248° C.
According to Plastic Additive Handbook, 5th edition, H. Zweifel, pp. 711, the barium salt of 5-phenyltetrazole begins to decompose at a temperature below 248° C., i.e. 240° C.
In the case of said prior art document, the decomposition of the barium salt of 5-phenyltetrazole occurs too early during the extrusion process and thus released gaseous products are not efficiently diluted and voids are not dispersed into the molten fluorinated polymer.
Therefore the composition with such chemical foaming agent cannot provide a chemically foamed fluorinated polymer with homogeneous aspect presenting good mechanical and electrical properties due to the difficulty of controlling the foaming process.